it may be loose already if not you should see some kind of hose clamp like affair.

the tabs that are behind the white are for centering the pic, leave those be for now, just turn the whole thing a few degrees CCW (look at it from behind), then turn on the set and see if that does it. You can do it with it on, but again you have to be careful as you can get a nasty shock working on sets with the back off.

It's that whole assembly, the green and white part. Watch out for the big red wire, if it has cracks in it, it will bite hard!

Wait a sec, what's that big red frayed looking wire sitting on top of the CRT?

looks like a wire tie, prob for wires for radios/speakers.